Chemistry Reference
In-Depth Information
will be slowly decomposed and some of it will leave the reactor unconsumed.
However, the activation energy for initiator decomposition exceeds that for con-
sumption of monomer (Section 8.16.1), and the initiator can be entirely decom-
posed at low monomer conversions if the wall temperature is too high for the
particular reaction system
[2]
.
Problems from inadequate mixing in tubular vessels can be alleviated to some
extent by inserting stationary mixing sections in the reaction train. To visualize a
stationary mixer, imagine a long strip of metal that is alternately twisted 180
in
clockwise and counterclockwise directions. Fluid flowing through the resulting
path is repeatedly divided, subdivided, and recombined, and the result is efficient
distributive mixing of volume elements that were originally far apart. The costs
of operation of such devices include increased flow resistance and the danger of
fouling or plugging.
The only important current application of tubular reactors in polymer syntheses
is in the production of high-pressure, low-density polyethylene. In tubular pro-
cesses, the newer reactors typically have inside diameters about 2.5 cm and lengths
of the order of 1 km. Ethylene, a free-radical initiator, and a chain transfer agent
are injected at the tube inlet and sometimes downstream as well. The high heat of
polymerization causes nonisothermal conditions with the temperature increasing
toward the tube center and away from the inlet. A typical axial temperature profile
peaks some distance down the tube where the bulk of the initiator has been con-
sumed. The reactors are operated at 200
300
C and 2000
3000 atm pressure.
The ethylene and polyethylene leave the reactor and pass into a primary sepa-
ration vessel which operates at a much lower pressure than the reactor itself.
Most of the ethylene (and any comonomer) is flashed off in this unit and recycled
through compressors to the tube inlet. Conversion per pass is of the order of 30%
with ethylene flow rates about 40,000 kg/h.
In many cases the reactor exit valve is opened and partially closed periodically
to impose a pressure and flow pulse that helps keep the tube from plugging with
polymer. Substantial pressure fluctuations occur in the reactor with this mode of
operation.
The other major reactor type used for high-pressure free-radical polymerization
of ethylene is a stirred autoclave type. There are very many variations and modifi-
cations of this type, as there are of tubular reactors. Stirred autoclaves usually have
length-to-diameter ratios of about 20. If they are well agitated with good end-to-
end mixing the reactor will approximate a CSTR. In many cases, however, a high
degree of directional flow is imposed and mixing is restricted by baffles so that the
autoclave operates more like a tubular vessel. Molecular weight and branching dis-
tributions are strongly affected by the mode of operation of polyethylene reactors.
12.6.3
Continuous Stirred Tank Reactors
An ideal CSTR is deliberately backmixed, in contrast to a tubular reactor where
plug flow and zero backmixing are ideal concepts. The feed is assumed to blend
